Development and application of potentiometric methods of charcterizing potassium in soils and micaceous minerals

This study was initiated to (1) establish the electrochemical cell requirements and procedures for the determination of extractable soil potassium with potassium ion-selective electrodes (K-ISEs), (2) evaluate K-ISE methods of determining exchangeable potassium in soil extracts, and (3) develop new K-ISEs for the determination of tetraphenylboron (TPB) extractable potassium in 60% acetone solutions;Electrochemical cells for the determination of potassium ion activities consist of a K-ISE and a reference electrode that contacts the test solution either directly or through a salt bridge. The suitability of using cells without transference for potassium determinations in systems where the ion causing the reference electrode response is maintained at a constant level was verified. For routine analytical work, however, a cell with transference consisting of a valinomycin-based K-selective electrode (VKE) and a sleeve-type double-junction reference electrode with a saturated lithium acetate salt bridge provided the best results;The VKEs circumvent the problems of interference from all the ions involved in soil potassium determinations except NH(,4)(\u27+). Difficulties due to NH(,4)(\u27+) were minimized by using barium chloride as the extractant and/or titrating the extracts to pH 11.6-11.8 with lithium hydroxide and a standard additions method of analysis. Determinations of Ba- and NH(,4)-exchangeable potassium in 30 soil samples were performed. The Ba- and NH(,4)-exchangeable potassium values were highly correlated (r = 0.986) although the barium values were slightly lower. The Ba-exchangeable potassium values obtained by direct potentiometry with the VKE and a solution pH of 11.6 were very highly correlated (r = 0.998) with and not significantly different ((alpha) = 0.01) from those obtained by AAS. The NH(,4)-exchangeable potassium values obtained with a modified standard additions technique and a solution pH of 11.6 were highly correlated (r = 0.987) with and not significantly different ((alpha) = 0.05) from those obtained by AAS;A simple procedure for the rapid construction of inexpensive K-ISEs based on valinomycin was developed and used to assess the suitability of electrode membranes with different compositions. This procedure yielded tube-mounted membrane electrodes that performed as well as commercially available potassium-selective electrodes in terms of their response characteristics and practical applications with soil extracts